1. Field of the Invention
The present invention relates to novel asymmetric carbonate compounds and, more specifically, it relates to novel carbonate compounds useful as solvents having excellent anti-oxidation properties, starting materials for various organic synthesis, pharmaceuticals, agricultural chemicals and the like.
The present invention further relates to a non-aqueous electrolytic solution comprising carbonate compounds and a non-aqueous electrolyte battery utilizing the solution.
2. Related Art
The term "carbonate compound" herein used means a diester compound of carbonic acid represented by the formula R--O--CO--O--R'. As such a carbonate compound, generally known are symmetric carbonate compounds such as dimethyl carbonate and diethyl carbonate, and cyclic carbonate compounds such as ethylene carbonate and propylene carbonate. These carbonate compounds are used as special solvents in the fields of pharmaceutical chemistry, agricultural chemistry and the like. And these carbonates are also used as starting materials and intermediates of dyes, plant protective agents, synthetic resins, pharmaceuticals, agricultural chemicals, and the like (for example, Japanese Patent Application Laid-open (KOKAI) Nos. 54-125617 and 54-63023). Further, it has been reported that electrolyte solvents comprising cyclic carbonate compounds are utilized for non-aqueous batteries containing alkali metals, alkaline earth metals or compounds containing these metals as cathode active materials, because of its high dielectric constant and good solubility as to inorganic materials of the carbonate compounds.
Though fluorine-containing carbonate compounds have not known well, some of such carbonate compounds have been reported and examples thereof are di-trifluoroethyl carbonates as starting materials of synthetic resins (U.S. Pat. No. 4,349,488), di-hexafluoroisopropyl carbonate and ethyl hexafluoroisopropyl carbonate as agricultural chemicals (U.S. Pat. No. 3,359,298), di(perfluorophenyl) carbonate as flame-retardants (U.S. Pat. No. 4,102,912) and the like.
Symmetric carbonate compounds are generally synthesized by a reaction of phosgen and an alcohol. On the other hand, for the synthesis of asymmetric carbonate compounds, it is necessary to introduce two alkoxy groups, and as the method for the synthesis of such carbonate compounds, there have been proposed a method by a reaction of a carbamate and an alcohol (Japanese Patent Application Laid-open (KOKAI) No. 57-28845), a method by allowing an alcohol to react with a chloroformate and the like.
Meanwhile, it is basically required for organic solvents to be able to dissolve organic materials and, in addition, they must be chemically and physically stable, can be a liquid state in a wide temperature range, have a low viscosity and have a high flash point, i.e., be difficult to catch fire. Further, depending on the purposes for which the solvents are used, it is also important that they have a high dielectric constant.
While ordinary organic solvents such as benzene, methanol and acetone generally have a low flash point and easily catch fire, the above described carbonate compounds have a relatively high flash point. For example, the flash points of dimethyl carbonate and diethyl carbonate are 22.degree. C. and 25.degree. C., respectively and hence they do not easily catch fire. Moreover, they have excellent properties as a solvent, for example, they can dissolve organic materials sufficiently, are chemically and physically stable and have a high dielectric constant. For these reasons, there are already proposed electrolyte solvents for batteries comprising carbonate compounds (Japanese Patent Application Laid-open (KOKAI) No. 61-64082).
As described above, the symmetric carbonate compounds whose alkyl groups consisting of carbon atoms and hydrogen atoms have more excellent properties as compared with the ordinary solvents. However, it cannot be said that they have sufficient characteristics for certain uses where the solvents are required to have chemical and physical stability, high flash point and high flame retardant properties such as uses in batteries, and electrolytic solutions for electrochemical reactions. In addition, compounds having a symmetric structure are generally prone to crystallize and have a relatively high melting point despite the low molecular weight (e.g., dimethyl carbonate has a melting point of 3.degree. to 4.degree. C.), and this leads to a drawback that the temperature range where they can be used as a liquid is limited.
The object of the present invention is to provide novel asymmetric carbonate compounds and, more specifically, the object of the present invention is to provide novel carbonate compounds which are chemically and physically stable, have a high dielectric constant, can sufficiently dissolve organic and inorganic materials and, further, useful as solvents having a high flash point and a low melting point.
Electric cells utilizing non-aqueous electrolytic solution have been conventionally and widely used as electric sources of various kinds of consumer electronic equipments because of their reliability such as storage characteristics. However, non-aqueous electrolytic solutions generally have electro-conductivity tens to hundreds times lower than aqueous electrolytic solutions and, in particular, non-aqueous electrolytic solutions having low decomposition voltage show poor charge/discharge cycle characteristics and short life-time in batteries. Further, non-aqueous electrolytic solution have another drawback, that is, when charge/discharge cycles are repeated in a non-aqueous electrolyte battery, needlelike metals, so-called "dendrites", are occasionally deposited and they are prone to be released from electrodes to form reactive metal powder, or to penetrate the separator separating the cathode and the anode to cause short circuit.
In order to improve the electro-conductivity of non-aqueous electrolytic solutions, it have been proposed that a low-viscosity solvent such as dimethoxyethane, tetrahydrofuran and 1,3-dioxoran is added to a solvent having a high dielectric constant such as propylene carbonate, .gamma.-butyrolactone and sulfolane (for example, Denki Kagaku (Electrochemistry), 53, No. 3, p. 173 (1985)). Further, it have also been proposed to, as attempts to improve the durability of electrolytic solutions, use a carbonate having a high decomposition voltage such as diethyl carbonate instead of a solvent having a low decomposition voltage such as dimethoxyethane to improve the battery charge/discharge efficiency (for example, Japanese Patent Application Laid-open (KOKAI) No. 2-10666), or add a phosphate to electrolytic solutions to make the solutions self-extinguishing (Japanese Patent Application Laid-open (KOKAI) No. 4-184870).
Meanwhile, because batteries having a high energy density are desired, various researches concerning high voltage batteries are being conducted. For example, researches have been conducted as to a battery comprising a cathode of complexed oxide of lithium and transition metal such as LiCoO.sub.2, LiNiO.sub.2 and LiMn.sub.2 O.sub.4 and an anode of metallic lithium, lithium alloys or a lithium/carbon compound and capable of generating voltage of 4 V. In such a battery, decomposition of electrolytic solutions due to oxidation become likely to occur and, therefore, conventionally used esters such as .gamma.-butyrolactone and ethyl acetate and ethers such as 1,3-dioxoran, tetrahydrofuran and dimethoxyethane are not preferred solvents since they have a low decomposition voltage and react with the cathode and, when such a conventional solvent is used in the battery of this type, it has further drawbacks, for example, the battery capacity is reduced every charge/discharge cycle and gas is generated to elevate the internal pressure of the battery. Therefore, electrolyte solvents having anti-oxidation properties have been desired.
When metallic lithium, a lithium alloy or a complexed oxide is used for the anode of the battery, metallic lithium deposited during the charging or deposited due to overcharging is highly reactive and there is possibility that it reacts even with a electrolyte solvent excellent in anti-oxidation properties. Further, another drawback of batteries of this type has been pointed out from the viewpoint of use, that is, when charge/discharge cycles are repeated in the battery, needlelike lithium crystals, so-called "dendrite", are occasionally deposited and they are prone to be released from electrodes to form reactive lithium powder or to penetrate the separator which separating the cathode and the anode to cause short circuit.
The present invention has been completed to solve the above described problems and its object is to provide a non-aqueous electrolytic solution, which is excellent in decomposition voltage and charge/discharge cycle characteristics and has a high flash point, i.e., which is suitable for an electrolyte solvent. Further, it is also the object of the present invention to provide a non-aqueous electrolyte battery which is capable of generating high voltage and excellent in battery characteristics.